Cnapter 6 



THE GENETICS OF SPONTANEOUS TUMOR 

 FORMATION 



By Clarence C. Little, Roscoe B. Jackson Memorial Laboratory. 



Introduction, 248. Mammary epithelial tumors, 251. Evidence that the tendency 

 to form such tumors is not due to the action of a single recessive gene, 251. Evidence 

 of an extra-chromosomal influence, 251. Evidence that at least part of the extra- 

 chromosomal influence can be transferred by foster nursing, 257. Evidence that 

 genetic factors also influence the incidence of breast tumors, 259. Evidence that 

 hormonal influences also affect the incidence of breast tumors, 259. Evidence that 

 coat color may play a part in influencing the incidence of mammary tumors, 260. 

 Summary, 261. Epithelial lung tumors, 261. Absence of extra-chromosomal influ- 

 ence, 262. "Dominant" nature, 262. Hormonal influences, 264. Coat color, 264. 

 Non -epithelial tumors, 264. Absence of extra-chromosomal influence, 266. Relation 

 of incidence to age, 268. Relation of incidence to sex, 268. Relation of incidence to 

 coat color, 269. Relation of incidence to hybridization, 269. Leukemias, 270. 

 Evidence of extra-chromosomal influence, 273. Comparison of the four general types 

 of neoplasms, 273. Bibliography, 273. 



Introduction 



The application of genetic methods in the analysis of the incidence of 

 spontaneous tumors in mice is a matter of considerable complexity. Various 

 factors and influences serve to modify the actual effects of genes. Yet in 

 spite of this fact there is compelling evidence that the genetic constitution 

 of an organism plays a part in determining whether or not it will develop a 

 tumor or tumors. 



The earliest work which contributed to this conclusion was that of Tyzzer 

 (loi), J. A. Murray (73), Bashford (8), Haaland (43") and Loeb (48). By 

 191 2 these investigators had independently demonstrated that families and 

 strains of mice differed in their capacity for producing breast tumors. 



A good deal of this earlier work lacked certain qualities which would 

 have greatly increased its accuracy. In some cases histological diagnosis of 

 the palpated nodules was absent. In others the number of individuals 

 studied was none too large. In some, knowledge of the more remote 



248 



